1. Field of the Invention
The invention generally relates to a secondary battery in which a plurality of battery cells are connected together in series. More particularly, the invention relates to a temperature abnormality detecting apparatus and method for a secondary battery mounted in a vehicle.
2. Description of the Related Art
Secondary batteries are typically used in vehicles that are driven either partially or entirely by an electric motor, such as electric vehicles, hybrid vehicles, and fuel cell vehicles. Electric vehicles are driven by an electric motor which runs using power stored in the secondary battery. Hybrid vehicles can be driven either by an electric motor which runs using power stored in the secondary battery, or by an engine, using the electric motor for additional power when necessary. Fuel cell vehicles can be driven by an electric motor which runs using power generated by a fuel cell, or by an electric motor which runs using power stored in a secondary battery as well as power generated by a fuel cell.
Because these secondary batteries require high-voltage and a great deal of power, a battery pack is used which is formed, for example, by connecting together in series approximately 30 battery modules, each of which has approximately six 1.2V or so nickel metal hydride battery cells connected to one another in series. This means that this kind of secondary battery would then ultimately have 180 (or 200 or more) battery cells connected in series.
While such secondary batteries are not mounted in vehicles that use only an internal combustion engine as the drive power source, they are essential for vehicles such as electric vehicles and hybrid vehicles. Further, while there was little need to consider cooling for lead batteries used to power auxiliary components (such as lights and audio systems) in vehicles, the cooling of these secondary batteries must be given due consideration.
In particular, it is necessary to detect the temperature of the secondary battery and cool it if necessary in order to inhibit problems from occurring, such as deterioration of the secondary battery due to high temperatures. As described above, however, the battery pack is formed of many battery cells connected together in series. Further, the battery temperature is detected for each battery cell. As a result, the battery temperatures of the battery cells may vary. For example, when the temperature of the secondary battery rises do to charging or discharging and cooling air is blown into the battery pack by a cooling fan, heat exchange with the battery cells located farther away from the cooling fan is done using the air that has already been used in the heat exchange with the battery cells that are closer to the cooling fan. Moreover, the area of each battery cell that comes into contact with the cooling air is not necessarily the same.
This is due to such factors as the arrangement of the battery cells in the battery pack, the placement of the cooling fan, and the location of the outside heat source. No matter what the cause, because the battery pack that is made up from multiple battery cells can not be cooled evenly, it is difficult to detect a temperature abnormality in the battery from the detected battery pack temperature (i.e., the temperatures of the plurality of battery cells). Technology related to the cooling of this kind of secondary battery is disclosed in the following publications.
Japanese Patent Application Laid-open No. JP-A-11-178231 discloses a temperature control device for a battery pack in an electric vehicle, which improves safety and usability of a high-voltage battery pack by precisely controlling the temperature of the battery pack. This temperature control device is provided with multiple battery temperature sensors inside a battery pack in which multiple battery cells connected together in series and/or in parallel are housed in a common case. The temperature control device is also provided with a signal processing portion which processes output signals from each of the battery temperature sensors. This signal processing portion has two circuits. One circuit determines that a signal from a battery temperature sensor having a low temperature resistance value is abnormal, and therefore allows the battery to charge or discharge, when only one of the battery temperature sensors has a low temperature resistance value that corresponds to a temperature that is lower, by a predetermined temperature or more, than the temperatures to which the low temperature resistance values of the rest of the battery temperature sensors correspond. The other circuit determines that the battery temperature is abnormal, and therefore prohibits the battery from charging or discharging, when some of the battery temperature sensors have high temperature resistance values that correspond to temperatures that are higher, by a predetermined temperature or more, than the temperatures to which the high temperature resistance values of the majority of the battery temperature sensors correspond.
Thus, when only one of the battery temperature sensors has a low temperature resistance value that corresponds to a temperature that is lower than the temperatures to which the low temperature resistance values of the rest of the battery temperature sensors correspond by a predetermined temperature or more, the temperature control device determines that there is an abnormality in the sensor, not the battery, so the output from that sensor is ignored and the battery is allowed to charge and discharge. That is, if only one of the battery temperature sensors inside the same battery case indicates a low temperature, no problems will be caused by the battery continuing to charge or discharge. Also, when some of the battery temperature sensors have high temperature resistance values that correspond to temperatures that are higher, by a predetermined temperature or more, than the temperatures to which the high temperature resistance values of the majority of the battery temperature sensors correspond, the temperature control device determines that the battery temperature is abnormal, and therefore prohibits the battery from charging or discharging. Ultimately, in a case when the temperature of each portion of the battery within the battery case is monitored using separate sensors, the temperature of part of the battery is actually abnormally high when the temperatures indicated by some of the sensors are higher than the temperatures indicated by the majority of the sensors. Against of this case, it is not necessary to prohibit the battery from charging or discharging when there is any other variance in temperature. Accordingly, this kind of temperature control enables unnecessary prohibiting of charging and discharging to be avoided.
Japanese Patent Application Laid-open No. JP-A-2001-313092 discloses a cooling device which brings a secondary battery into a suitable temperature range and detects an abnormality in a cooling device of the secondary battery. This cooling device includes two circuits. One circuit inhibits an abnormal increase in the temperature of the secondary battery by driving a fan for cooling in Hi mode when a difference ΔT between an actual battery temperature Tb and an estimated temperature Te calculated from a charge-discharge current of the secondary battery and the cooling capability is greater than a threshold value Tr. The other circuit determines that some sort of abnormality has occurred with the cooling function of the device and outputs a signal indicative of an abnormality when the difference ΔT remains greater than the threshold value Tr after the cooling fan has been driven in the Hi mode for a predetermined period of time.
This cooling device both enables an abnormal temperature increase in the secondary battery to be suppressed, as well as enables an abnormality with the cooling function of the device to be detected more appropriately.
The temperature control device disclosed in Japanese Patent Application Laid-open No. JP-A-11-178231, however, may inappropriately detect a temperature abnormality when there is a difference in the cooling efficiency resulting from a difference in the location of the battery cells (i.e., due to, for example, the positional relationship between the battery cells in the battery pack and the cooling fan) even if the battery itself is functioning normally. That is, the temperature control device determines that the battery temperature is abnormal even when a variance in the battery temperature occurs due to the location of the battery cells.
Also, the cooling device disclosed in Japanese Patent Application Laid-open No. JP-A-2001-313092 requires that the capability of the cooling fan at each of three levels (Lo, Me, Hi) be grasped in advance in order to detect an abnormality in the cooling device. Further, an abnormality in the cooling device is only able to be detected when the cooling fan is in one of these three modes (i.e., levels or stages). With this cooling device it is difficult to determine an abnormality when the operating state of the cooling fan is being changed continuously (e.g., when the operating voltage is being changed continuously) instead of in stages. In particular, this cooling device requires that coefficients be set beforehand to calculate the estimated temperature. Setting these coefficients is difficult, however, because they differ with each vehicle and each battery.